In many situations, it is desirable to limit the speed of an engine and this limiting may be accomplished hydraulically when the engine is driving a pump. More specifically, the speed of an engine driving a pump may be controlled to prevent overspeed by increasing pressure in a line supplied by the pump to impose a load torque on the engine which reduces engine speed.
Currently, many dozer or loader tractors are driven by a hydrostatic transmission. In order for the pump and motor of the hydrostatic transmission to operate near their normal design limits, there must be some assurance that unexpected or infrequent operating conditions will not cause the speed of the pump or motor of the hydrostatic transmission to exceed maximum safe design limits. Such unexpected or infrequent operating conditions could include an externally applied load, such as gravity effects on the tractor on a steep hill, or pushing effects of other vehicles. When such externally applied load causes an increased speed of the tractor and, thus, of the prime mover engine coupled to the hydrostatic transmission, there is a possibility of exceeding maximum safe design limits for the pump and motor of the transmission.